He’s SpongeBob, a fun-loving but often clueless and immature fellow who lives at the bottom of the sea and wears square pants. She’s a sea-diving squirrel named Sandy Cheeks, SpongeBob’s intelligent and level-headed pal.

Re-united in a show for the first time since they acted together at Vassar, Ethan Slater ’14 and Lilli Cooper ’12 are headliners of the Broadway cast of SpongeBob SquarePants, a rollicking tribute to the beloved Nickelodeon character who has been entertaining millions of young fans since 1999.

Lilli Cooper ’12 and Ethan Slater ’14 at Times Square during a rehearsal breakPhoto: Samuel Stuart

Some of those fans are notable musicians—Joe Perry and Steven Tyler of Aerosmith, Sara Bareilles, Cyndi Lauper, John Legend, and the late David Bowie, to name a few—who wrote the songs that Slater and Cooper and the rest of the cast will perform in the show. Previews begin Nov. 6 at the Palace Theater in Manhattan, and SpongeBob’s official Broadway debut is Dec. 4.

It’s Slater’s first Broadway appearance—Cooper made her debut in Spring Awakening when she was still in high school and also performed in Wicked—and both say they’re looking forward to bringing SpongeBob’s wacky adventures to New York after a series of performances last year in Chicago.

SpongeBob drew rave reviews from Chicago critics (“Wildly playful and imaginative...”- Chicago Sun Times; “Deliciously quirky...” - Chicago Tribune), but Slater says the show has undergone some significant improvements since rehearsals for the Broadway production began in New York this summer. “Some of the music has changed and the jokes are better,” he says. “The play continues to evolve.”

Slater has been witnessing the evolution of SpongeBob SquarePants since its inception. In the summer following his sophomore year at Vassar, he was one of the original actors invited by director Tina Landau to a “movement lab,’’ a series of workshops where participants brainstormed about turning the SpongeBob saga into a play. “At that point, there was no script, no music; it was just an idea,” Slater says.

Over the next several months a script was developed, some songs for the show were written, and Landau began to hold auditions. Slater was cast as SpongeBob, and he was in the room with Landau for musical auditions when he looked at a list of names of those who would be singing that day. “I flipped the page, and there was Lilli’s name,” he recalls. “When she walked in, it was like a reunion. We were really excited to see each other.”

Slater and Cooper say they had followed each other’s careers after they graduated, but it was the first time they’d met up professionally. Cooper, who grew up in a theatrical family—her father, Chuck Cooper, won a Tony award in 1999—says she chose Vassar “because it had such a strong drama and film department and because it was close to New York City but far enough away that I could have a regular college experience.” She was a member of Vassar Repertory Dance Theater (VRDT) and took dance classes with her first-year advisor, Senior Dance Lecturer Katherine Wildberger. “Kathy was a big influence in my involvement in the arts,” Cooper says, “I loved performing at Vassar; the production value was always fantastic, and the caliber of show selection was always so high.”

Slater credits Drama Prof. Chris Grabowski with steering him to the SpongeBob project. Shortly before he was tapped to be a part of the movement lab with Landau, he had landed a role in a production of Romeo and Juliet. Unsure which path he should choose, Slater contacted Grabowski and explained his dilemma.

Sandy Cheeks and SpongeBob in performance in Chicago where the show first opened

Grabowski told him the choice was obvious, Slater recalls. “Chris told me, ‘Shakespeare will always be there, but if you turn down a chance to work with a director like Tina Landau, you’ll always regret it,’” he says.

Grabowski recalls the conversation a little differently: “I think I said, ‘Ethan, are you crazy? This is a no-brainer!’ And now, a few years later, it’s nice to be right.”

Grabowski directed Slater and Cooper in The Skin of Our Teeth in 2011. Cooper had the female lead (“Lilli could play any part in any play—she was already a star,” Grabowski says), while Slater had a smaller role as a telegraph boy.

“Tina told us that if we can make people laugh for a couple of hours, that’s certainly worth something—it’s what the world needs right now.”

Grabowski and Slater later worked together in a production of the 1930s musical The Cradle Will Rock, and Grabowski says Slater’s talent was obvious. “Ethan was amazing, a beacon of light with an incredibly open face,” Grabowski says. “He was absolutely uninhibited and knew how to project. He acted on a scale that’s challenging to most young actors.”

Slater has been able to support himself through acting for most of the time since he graduated, “although I also worked in a coffee shop.” He had a lead role in Lightning Bugs in a Jar, a short film written by Vassar alum Ian Simpson ’05 and shot during a residency at the Catwalk Institute, an artist retreat founded by Purcell Scheu Palmer '62. He's also had roles in Baghdaddy off-Broadway and in a musical version of Diner with the Delaware Theater Company, and he is a performer in the Web series, “Redheads Anonymous” and “New Mayor of New York.” Slater is also a member of the ensemble cast of One Year Lease, a Manhattan-based theater company directed by Vassar alum Ianthe Demos ’00.

As the show ramps up for opening night, Slater and Cooper say the hours are long and the work is physically demanding. “I trained with a contortionist to prepare for the show,” Slater says. “I spent six months in the gym before rehearsals began and took dance classes five days a week. I do a lot of stretching before rehearsal every day. Doing the show is a real cross-fit workout.”

It’s a rigorous routine, but Slater says he’s enjoying every minute. “I love the camaraderie of the whole cast and crew, and it was totally surreal to see the names of the people who wrote the music,” he says. “But at the same time, they did it because they’re all fans of the show.”

Previews for SpongeBob SquarePants begin on Nov. 6, with the official opening scheduled for Dec. 4.

Cooper agreed. “It’s not often you are on a stage that’s totally ego-less, and that’s a tribute to Tina (Landau),” she says.

Slater says the enormity of being a headliner on Broadway really hasn’t hit him yet. “I was sitting in the theater the other day with my friend Danny Skinner, who plays Patrick in the show, and we were saying it still doesn’t feel real,” he says. “It’s sort of an otherworldly experience.”

As wacky and silly as the show may be, Slater says he’s genuinely humbled to be a part of it. “At Vassar we were taught, ‘Theater means something; there’s a reason a play was written and why it’s performed,’” he says. “And yes, SpongeBob is silly, but it means something, too. There’s a message.

“Tina told us that if we can make people laugh for a couple of hours, that’s certainly worth something—it’s what the world needs right now,” Slater adds. “I’ve never been prouder of a show in my life. I am thrilled to be making my Broadway debut as SpongeBob.”